Behind the Forecast: Thundersnow? Here’s what you need to know

LOUISVILLE, Ky. (WAVE) - Thundersnow is a rare weather event that tends to get significant attention when it transpires.

Thundersnow happens when lightning occurs in a snowstorm. Strong instability, abundant moisture, and a lifting mechanism (like a warm front) are typically prevalent in situations that lead to thundersnow. In the atmosphere, we’ll see cold air sitting on top of warm, moist air. Upward motion, in a scenario like this, helps to produce convection.

Air is unstable if it rises on its own after being pushed up by a cold or warm front. Since the air is typically below freezing (and thus drier) throughout the atmosphere in winter snowstorms, the atmosphere is usually more stable. When shallow warm air layers can rise on their own after being lifted, snowfall increases and an electrical charge builds enough for lightning to occur.

Only the strongest winter storms produce thundersnow.

Thundersnow can develop in three ways.

Sometimes thundersnow forms like a typical thunderstorm along a powerful arctic cold front. Warm air bubbles and rises in the atmosphere, creating a thunderstorm with snow or snow squall. Cloud-to-ground lightning can come along with a strong cold front like this as it passes by.

Nor’easters can produce thundersnow as the counterclockwise rotation pushes picked of air higher in the atmosphere. Once the air climbs high enough, a sufficient charge builds for lightning to form. Human-made structures, like skyscrapers, can focus a positive charge in the lower levels of a snow cloud, concentrating enough of a charge for lightning to form.

In lake-effect snowstorms, especially off Lakes Ontario and Erie, thundersnow can also occur. Erie and Ontario are the shallowest Great Lakes, which allows them to warm more quickly during the summer months. Their water temperatures remain mild in the fall and early winter as air temperatures fall. The air warms as it travels over the lakes and then rises into the colder atmosphere. This creates vertical heat transfer, allowing for charge separations and thundersnow.

Thundersnow is common with heavy snowfall rates. Snowfall rates of two to four inches per hour can be seen with thundersnow.

On January 17th, 1994, a thundersnow storm dropped around two feet of snow in Louisville. A narrow band of thunder and lightning within the center of the storm traveled right over the city where the highest accumulations were seen.

Thundersnow lightning is infrequent compared to summer thunderstorms; it is also usually cloud-to-cloud and not cloud-to-ground. Lightning is brighter and blue due to the light reflecting off snowflakes and ice crystals.

Thundersnow is more muffled due to heavy snowfall. Instead of the typical loud cracking sound, thunder within thundersnow sounds more like a low rumble. Thunder can typically be heard around 10 miles away from a lightning strike. Within the thundersnow, thunder can only be heard within two to three miles of the lightning strike.

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